The present invention relates to a suspension system for a vehicle, such as a heavy duty tractor.
Heavy duty tractors have been provided with front axle hydraulic suspension systems which include a pair of hydraulic cylinders connected between the frame and the axle of the tractor. One such system is described in German patent number DE 4242448, published Mar. 31, 1994. Such suspension systems also include hydraulic accumulators and can perform a chassis leveling function. Such tractors are subject to a wide range of axle loads, and these loads can exceed the normal pressure range of the accumulator. If an implement is attached to such a tractor, implement operation can produce large changes in load and chassis attitude which can interfere with implement control and with chassis attitude control. Leveling response time can be slow because flow to both the head and rod ends of the cylinders must be controlled by a pair of solenoid operated valves.
Such systems use a fixed pressure regulator to control rod side pressure or uses a common head and rod pressure. To achieve the maximum load range it is desirable to have a high rod pressure when the head pressure is low and a low rod pressure when the head pressure is high. To maintain level tractor attitude and better implement control it is desirable to restrict vertical chassis displacement when significant changes are anticipated. It is not desirable to block flow on both sides due to the potential to work head and rod against each other due to flow into the cylinder or thermal effects.
Such prior systems also do not allow for independent rod and head charging. As a result, flow typically goes to the rod side first, which is the path of least resistance. This results in delayed leveling operation, and results in the discharge of rod side oil that was just introduced into the rod side, thus wasting energy.
Accordingly, an object of this invention is to provide a hydraulic suspension control system which can accommodate a wide range of loads.
A further object of the invention is to provide such a system which avoids delayed leveling operation.
A further object of the invention is to provide such a system which does not waste energy by discharging rod side oil.
A further object of the invention is to provide such a system which maintains level tractor attitude and which improves implement control.
These and other objects are achieved by the present invention, wherein a hydraulic control circuit is provided for a vehicle suspension system having a fluid pump and suspension cylinders, each having a head end and a rod end. The control circuit includes a proportional rod pressure control valve with a pair of pilots. The first pilot is exposed to a pressure which is communicated to the rod end of the suspension cylinder. The second pilot is exposed to pressure which is communicated to the head end of the suspension cylinder. The rod pressure control valve controls pressure in the rod end of the suspension cylinder as a function of and proportional to the pressure in the head end of the suspension cylinder. System pressure is communicated to the rod pressure control valve via a solenoid operated raise/lower valve. In order to achieve the maximum load range, the system produces a high rod pressure when the head pressure is low and produces a low rod pressure when the head pressure is high.
Vertical chassis displacement is restricted to maintain level tractor attitude and provide better implement control by restricting flow between the cylinders and accumulator on one side of the suspension. Pilot pressure is applied to the damping valve rapidly to respond quickly to load changes. Flow between the cylinders and accumulator may be restricted or completely blocked. This results in a significant pressure differential between the cylinders and the accumulator. To control pressure equalization, pilot pressure is metered off across an orifice. The metering opens the damping valve slow enough so that the controller may respond to position changes by leveling, closing the pilot exhaust valve to hold the damping valve in position or reapplying pilot pressure to start the opening process again.
As an alternative to completely blocking flow between cylinders and accumulators, it is desirable to orifice flow between cylinders and accumulators. Gradual pressure equalization can easily be made up by leveling flow. When pressure is partially or completely taken care of by the equalization orifice, control requirements are simplified and the transition from damped to undamped operation is quick and seamless. The equalization orifice also improves safety by insuring that the cylinders and accumulators can be drained by the same elements, and insures equalization if the tractor is shut off while the suspension is in damped condition, thus avoiding unexpected motion if the damping valve opens at a later time.
The head and rod ends may be independently charged in order to achieve prompt leveling operation, and in order to avoid the discharge of rod side oil, thus saving energy. Flow can be applied to the head side first to accomplish a portion of the leveling before flow/pressure is applied to the rod side to set the rod side pressure. For this purpose, the head and rod side of the system are cross piloted.